The present invention is directed to magnetic resonance imaging (MRI) apparatus and methods, and particularly to apparatus and methods which increase the accuracy and/or resolution of MRI images, and/or decrease the time required to obtain an MRI image.
MRI is based on solving the Bloch Equations EQU dM.sub.x /dt=-.gamma.H.sub.z M.sub.y -M.sub.x /T.sub.2, (1.3.3) EQU dM.sub.y /dt=-.gamma.H.sub.z M.sub.x -M.sub.y /T.sub.2, (1.3.2)
and EQU dM.sub.z /dt=-(M.sub.0 -M.sub.z)/T.sub.1, (1.3.3)
which gives the magnetization M=M.sub.x x+M.sub.y y+M.sub.z z of magnetic nuclei (typically protons) in the presence of a magnetic field H=H.sub.z z. Standard nuclear magnetic resonance (NMR) uses a homogeneous magnetic field, but to obtain the spatial resolution required for image mapping, gradient magnetic fields must be added. The interaction of the magnetic spins with the environment is given in terms of the transverse relaxation time T.sub.2 which determines the rate of decay of the M.sub.x and M.sub.y components of the magnetization M to zero, and the longitudinal relaxation time T.sub.1 which determines the rate of decay of the M.sub.z component of the magnetization M to the value M.sub.0. It has been previously shown (Prolongation of Proton Spin Lattice Relaxation Times in Regionally Ischemic Tissue from Dog Hearts, E. S. Williams, J. I. Kaplan, F. Thatcher, G. Zimmerman and S. B. Knoebel, Journal of Nuclear Medicine, May 1980, Vol. 21, No. 5) that T.sub.1 is different in normal and ischemic heart muscle by about 10%. Using the inversion recovery technique" (which is described in detail in U.S. Pat. No. 4,383,219, entitled Nuclear Magnetic Resonance Spatial Mapping, issued May 10, 1983 to Jerome I. Kaplan, and is incorporated herein by reference) the 10% difference in the relaxation times can be amplified into a 25% differentiation in the magnetization in the normal and ischemic regions of the heart. Thus mapping this difference in magnetization allows for a mapping of the normal and ischemic regions of the heart.
High-resolution MRI images of non-moving body parts are routinely obtained. However, an added difficulty with the heart arises from its large scale rapid motion (1 beat/second). To avoid blurring the MRI image must be obtained in less than 1/20th of a second.
As is shown in the present specification, standard mapping procedures do not permit a high-resolution for mapping times of less than 1/20th of a second. An advantage of the present invention is to obtain high-resolution images for mappings where the data is acquired in a short period of time.
The mapping procedure of the present invention is as follows:
1) A 90.degree. radio frequency pulse rotates the magnetization M from its equilibrium direction along the z-axis (i.e., the direction of the large applied static magnetic field) to the x-y plane; PA1 2) The magnetization M at position (x,y) precesses at a frequency .omega.(x,y) about the local gradient fields at position (x,y); PA1 3) The total magnetization is recorded by monitoring the induced voltage in receiver coils; and PA1 4) The magnetization is Fourier transformed to give an effective mapping object density .rho.*(x,y). PA1 5) A weighting function is obtained which combines in a specific manner an array of effective object density .rho.*(x,y) values to define corrected object density .rho.**(x,y) which more accurately represents the actual object density .rho.(x,y).
For long mapping times, the effective object density .rho.*(x,y) is an accurate map of the actual spin density .rho.(x,y) using this standard procedure. However, for short mapping times, the effective object density .rho.*(x,y) is not an accurate mapping of actual object density .rho.(x,y). The present invention therefore includes a further step:
This additional step allows for high-resolution mapping of a moving subject, such as an organ like the heart. Furthermore, this additional step can also be used to obtain an increase in resolution or a decrease in mapping time.
Therefore, it is an object of the present invention to provide apparatus and method for magnetic resonance imaging which increases the accuracy of magnetic resonance images.
It is another object of the present invention to provide apparatus and method for magnetic resonance imaging which improves the spatial resolution of magnetic resonance images.
It is another object of the present invention to provide apparatus and method for magnetic resonance imaging which decreases the time required to obtain magnetic resonance images.
It is another object of the present invention to provide a means for reducing the stabilization time of the magnetic field gradients.
Further objects and advantages of the present invention will become apparent from a consideration of the drawings and the ensuing detailed description.